Lithium ion battery is a secondary battery having characteristics that it has a high energy density, and enables a relatively high voltage, and thus, is widely used in small electronic equipment such as a laptop computer, a video camera, a digital camera, a mobile phone, and the like. In the future, it is also expected to be used as a power supply for large apparatuses such as an electric car, and a dispersed-type power supply for household use.
As a conventional positive electrode active material for using in a lithium ion battery, lithium composite oxides such as LiCoO2, LiNiO2 and LiMn2O4 are representative. Among them, LiMn2O4 having a spinel structure is noted as a positive electrode material for a lithium ion battery because it is superior in safety and advantageous in price due to the use of manganese as an abundant resource.
However, LiMn2O4 is problematic as it shows serious cycle degradation at high-temperatures and large deterioration in storage, and the capacity thereof is smaller than that of lithium cobalt oxide (LiCoO2 currently predominantly used, and therefore various technical improvements have been studied for overcoming these problems.
For example, it is known that a portion of Mn in LiMn2O4 is replaced with another element in order to improve the cycle property of LiMn2O4.
Japanese Patent Application Public Disclosure No. 11-189419 (Patent literature 1) discloses that trivalent metals such as Co, Cr, Al and the like is doped to 16d site in the lithium-manganese composite oxide having a spinel structure which can be represented by a chemical composition formula Li1+xMn2-yMyO4+z and that the dope of trivalent metal(s) is very effective to restrict the reduction of capacity to minimum.
Further, Japanese Patent Application Public Disclosure No. 11-265717 (Patent literature 2), discloses that the capacity maintenance rate and the cycle property at high-temperature can be both increased without a large reduction of initial discharge capacity by the addition and coordination of a very small amount of element having a lattice energy (power for stabilizing the lattice) higher than Mn, namely Al or transition metals other than Mn. Concretely, a spinel type lithium-manganese oxides consisting of a general formula LiMnx-yMyO4 (x: 1.8-2.1, y: 0.01-0.1, M: Al, or transition metals other than Mn). It also discloses that the added element M is substituted at the rate of 0.01-0.1 with respect to Mnx (x: 1.8-2.1) because with M less than 0.01, the capacity maintenance rate and the property at high-temperature are not improved, on the other hand, with M more than 0.1, an initial discharge capacity is largely reduced and thus it is not practical.
Patent No. 3595734 (Patent literature 3) also discloses that an electron orbital(s) in crystal is changed by replacement of a portion of Mn with other element, and the crystal structure is reinforced by increasing the bonding power between Mn—O which forms the skeleton of the crystal.
The improvement of properties has been also proposed by noticing a specific surface area.
Patent literature 1 proposes to keep the specific surface area of lithium-manganese composite oxide at or less than 1.2 m2/g in order to increase the cycle lifetime.
Japanese Patent Application Public Disclosure No. 2002-226214 (Patent literature 4) proposes to keep BET specific surface area at or less than 1.0 m2/g from the viewpoint of the stability at high-temperature and the workability for production of an electrode.
Further, Patent literature 3 proposes to keep BET specific surface area at or less than 0.9 m2/g, especially at or less than 0.6 m2/g in the lithium-manganese oxide represented by Li1+xMn2-x-yAlyO4 (X≧0, y>0) from the viewpoint of the cycle and preservative properties at high-temperature.
The improvement of properties has been also proposed by noticing lattice constant.
For example, Patent literature 2 proposes a spinel type lithium-manganese oxide having the lattice constant at or less than 8.200-8.250 {acute over (Å)}. It is described that when the lattice constant is set within the range, even if an element(s) other than lithium and manganese is(are) added and coordinated, the oxide may have the lattice constant very approximate to the basic lattice constant of lithium-manganese spinel structure (8.240 {acute over (Å)}), and thus basic lattice defect may not occur.
Patent literature 3 discloses that the lattice constant should be equal to or smaller than 8.220 {acute over (Å)} in the above-described lithium-manganese oxide from the viewpoint of the cycle and preservative at high-temperature
Japanese Patent Application Public Disclosure No. 2004-265749 (Patent literature 5), it is proposed that the lattice constant of aluminum substituted lithium manganate should be equal to or less than 8.245 {acute over (Å)} in order to prevent the reduction of the rate of maintenance of the discharge capacity and the inability to keep the charged state for a long time.
There is also a prior document in which the improvement of properties is attempted by defining the distribution of particle size.
Patent literature 4, defines the average particle size of lithium-manganese oxides within 5-20 μm, because, with the average particle size less than 5 μm, the stability at high-temperature remarkably reduces, and further the workability for production of an electrode decreases, on the other hand, with the average particle size greater than 20 μm, high-rate charge and discharge properties remarkably reduce. In working example 1, it is described that the average particle size was 15.1 μm.
Patent No. 3856015 (Patent literature 6) discloses that when D10, D50 and D90 are defined as a particle size at which point the cumulative frequency of volume reaches 10%, 50% and 90% respectively, it is desirable to satisfying all of 0.1≦(D10/D50)≦1, 1<(D90/D50)≦3 and 5 μm≦D50≦40 μm. It is described that when the condition is satisfied, an electrode density may increase without imparing the improvement of the overdischarge property. In a working example 1 of Patent literature 6, it is described that D10, D50 and D90 were 10 μm, 13 μm and 18 μm, respectively, and further, in a working example 2, it is described that D10, D50 and D90 were 9 μm, 12 μm and 16 μm, respectively.
Further, there is also a prior document in which a tap density is defined. For example, Japanese Patent Application Public Disclosure No. 2006-228604 (Patent literature 7) discloses a lithium-manganese-nickel-aluminum composite oxide having a tap density of 1-1.5 g/cm3. As the reason, it is described that with the tap density less than 1 g/cm3, the amount which can be loaded at production of a battery decreases, and thus a definite capacity cannot be filled up, and on the other hand, with the tap density over 1.5 g/cm3, the average particle diameter d50 excesses 12 μm and therefore the coating onto the surface of current collector cannot be performed evenly.    [Patent literature 1] Japanese Patent Application Public Disclosure No. 11-189419    [Patent literature 2] Japanese Patent Application Public Disclosure No. 11-265717    [Patent literature 3] Japanese Patent No. 3595734    [Patent literature 4] Japanese Patent Appln Public Disclosure No. 2002-226214    [Patent literature 5] Japanese Patent Appln Public Disclosure No. 2004-265749    [Patent literature 6] Japanese Patent No. 3856015    [Patent literature 7] Japanese Patent Appln Public Disclosure No. 2006-228604